1. Field
The invention is in the field of semiconductor devices, more specifically pertaining to start up circuits for switched capacitor networks that are used in semiconductor device power delivery.
2. Related Art
Presently, power delivery to most integrated circuits (ICs), such as processors, graphic and central processing units (CPUs), memory, and input/output (I/O) interfaces within an electronic device, such as a computer, relies on a buck voltage regulator. The voltage regulator provides different voltages and different amounts of available current to different components of the electronic device.
As electronic devices trend toward power reduction and simultaneous integration of more features, the power delivery network must provide power at a number of different voltage levels for different components. For example, as depicted in FIG. 1A, a mobile computing device 100 includes a motherboard 105 which supports a chipset requiring at least three different regulated voltage rails. This is achieved in stages. In the first stage, a voltage regulator (VR) 110 regulates a power supply, such as i-ion cell(s) 106 to a first voltage level of 3V. Any other power supply may be used in place of the Li-ion battery including rectifying power supply connected to alternating current (AC) mains power supply to produce a 12V or less output voltage. This output voltage is used as input to VR 110.
The VR 110 accommodates a range of voltages that Li-ion battery or mains voltage may output throughout various levels of discharge (e.g. 4.2V-4.8V/cell). As further shown, point of load (POL) voltage regulators coupled with the VR 110 receive the 3V supply as input and provide a second stage of power delivery for each of any number of output load circuits on the motherboard 105. In the example of FIG. 1A, the memory VR 111 provides a 1.3V rail to memory 140, the disk drive VR 112 provides a 1.75V rail to a Serial Advanced Technology Attachment (SATA/100, SATA/300, SATA/600 etc.) compliant interface 145, and the peripheral bus VR 113 provides 2.3V to a PCI Express chip set (PCIe v1.1, PCIe 2.0, etc.) 150.
The three voltages mentioned above are provided only as examples, and a typical computer system may have additional loads that require additional voltages. With additional POL voltage regulators potentially required for a Universal Serial Bus (USB 1.1, 2.0, 3.0, etc.), legacy I/O, wireless interfaces, a CPU, etc., the area of motherboard 105 consumed by voltage regulators in some cases may approach 40%.
A common buck voltage regulator usually has low efficiency at light loads and only performs well at high load. In ultra-mobile computing platforms and in energy-saving fixed platforms, loads are being reduced, and buck-type regulators may frequently operate well below maximum efficiency. Switched capacitor dividers are therefore proposed as a replacement for or supplement to buck converters, linear regulators and other types of voltage regulators.
Switched capacitor circuits use power switches and capacitors to regulate power. Initially at startup, the capacitors have no charge, but must be charged when the circuit is powered on in order to operate. During startup, a large rush of current into the uncharged capacitors can damage both the power switches and the capacitors. If startup is uncontrolled, over-voltages larger the typical component specifications can easily occur.
The over-voltages can be reduced by adequately matching the components in the circuit. If the capacitors are exactly equal, then the charge and the voltage increases will distribute equally. However, a sufficiently accurate match is expensive to achieve in mass produced components. In addition, matching the capacitors does not help gate drive schemes that use the internal capacitor voltages.